One possibility is that 28-30 Ohms is what you get (for "free") with many
MOS fabrication lines. On many, many PCBs we have to build out the PCB
trace impedance to 50 Ohms with a series source terminating resistor.

These series terminators work, but at a cost in propagation delay and drive
capability.

Facing up to the fact that you have 28-ohm drivers, and adjusting the PCB
trace dimensions appropriately, saves you a lot of parts and headaches. We
do it all the time on non-Rambus traces, and 28 ohms seems to come up
pretty regularly in IBIS models of drivers!

Larry Miller

At 12:40 PM 5/5/00 EST, you wrote:
>Hi Gurus,
>
>One thing for sure is that larger characteristic impedance means thinnner
>trace on PCB which also means more fringing electric and magnetic fields. So
>more emission and susceptibility. Definitely space is something to be
>concerned.
>
>So what value of impedance you choose is determined by the above and the
>fact that the smaller characteristics impedance (thicker traces) require
>powerful drivers. So power is the an issue here.
>
>I do not know why Rambus choose the magic number of 28 Ohm. Does any body
>know why 28 Ohm in Rambus?
>
>Subas
>
>
>>Doug Brooks wrote:
>>
>> > OK, gurus....
>> >
>> > Lets say I have a trace on a board between two devices. There is nothing
>> > that determines what the characteristic impedance of the trace should
>>be.
>> > That is, there are no device considerations, no space considerations, no
>> > power considerations, etc. that would favor one value of characteristic
>> > impedance over another. It is my opinion that as long as the
>>characteristic
>> > impedance is constant over the length and properly terminated, the
>>choice
>> > of impedance value in this situation is completely arbitrary.
>> >
>> > Does anyone (having accepted the assumptions above) have a different
>> > opinion? In particular, are there any EMI reasons that would favor one
>> > value of characteristic impedance over another? (But don't tell me the
>> > assumptions above are wrong! That's not the question!)
>> >
>> > Doug Brooks
>> > .
>> > ************************************************************
>> > See our updated message re in-house seminars on our web page
>> > .
>> > Doug Brooks, President doug@eskimo.com>> > UltraCAD Design, Inc. http://www.ultracad.com>> >
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